During lock stitch sewing, stitches are formed by a needle thread or threads, introduced from one side, interlacing with an underthread supplied from a bobbin on the other side. Typical lock stitch sewing results in strong seams with good strength and abrasion resistance, but has a disadvantage in the limited length of sewing that is possible before having to replace the underthread bobbin.
In this regard, the underthread is supplied or delivered from a bobbin which is located in a bobbin case. When performing lock stitch sewing, a commercial sewer will typically purchase pre-wound bobbins that are already wound with sewing thread in such a fashion that they can be placed inside the bobbin case for sewing. Pre-wound bobbins are conventionally supplied either with sidewalls (known in the art as "pre-wound sidewall bobbins") or without sides (known in the art as "pre-wound sideless bobbins").
Sidewall bobbins have a flange made typically from cardboard, plastic or paper. The thread is typically wound onto a flangeless core with the sidewalls (flanges) thereafter being attached to the core ends in a secondary operation. Alternatively, the thread may be wound onto a core with the sidewalls already attached. The former bobbin-winding technique allows the finished bobbin to be sized correctly by virtue of the pressure applied during the sidewall attachment process.
The reasons for having sidewalls on a bobbin include (i) preventing the yarn from looping under the bobbin Qr over the bobbin case post and subsequently breaking, (ii) controlling the thread draw-off tension, and (iii) acting as a braking mechanism to reduce thread overspinning or backlash when sewing stops. In addition to these functions, bobbin sidewalls typically have been thought to be required for proper performance on recently developed automated bobbin changing equipment.
Sideless bobbins have no sidewalls. In this regard, yarn is conventionally wound onto a cylindrical core in the production of pre-wound sideless bobbins. Unlike sidewall bobbins, the yarn wound on a sideless bobbin is tacked together to control the amount and uniformity of draw-off tension, prevent the yarn from looping under or over the bobbin and subsequently breaking, and control or reduce bobbin backlash or overspinning when sewing stops. A variety of techniques may be employed to tack the yarn on the sideless bobbins. For example, tacking the yarn together on the bobbin can be done by softening a bond that is on the yarn via heat or chemical reaction or simply applying a tacking agent, such as wax or other soft, tacky materials, to the yarn.
Underthread supplied from a bobbin has a significant impact on seam quality as well as sewing productivity. The correct amount and uniformity of bobbin draw-off tension throughout the entire bobbin is important to achieve seam quality and performance.
The amount of draw-off tension is largely controlled by loosening or tightening a leaf spring located on the bobbin case so as to responsively decrease or increase, respectively, the spring's contact force against the thread. Draw-off tension which is not set correctly at the leaf spring or which changes during the sewing operation will cause loose stitches (on top or bottom) thus creating a defective seam. It is recognized in this art that the leaf spring is most desirably set so as to cause the thread to exhibit the least amount of draw-off tension consistent with high quality stitches. Lesser leaf spring pressure force on the thread, and thus a lesser amount of resulting draw-off tension, is known to cause less thread degradation due to frictional abrasion by the spring (and thereby also lessen the potential for thread lint and other debris to build-up under the spring). In addition, a lesser amount of spring pressure will not exacerbate tension non-uniformity caused inherently by thread surface irregularities.
The uniformity of draw-off tension as the bobbin is unwinding can be controlled or influenced by many factors. These factors include bobbin sidewalls, uniformity of yarn or thread diameter and thread friction, amount and uniformity of tack as the bobbin unwinds, and the ability of the tension spring to maintain uniform pressure or tension on the yarn as it passes through the bobbin case tension spring. It is generally known in the art that a lower variation in bobbin draw-off tension will produce, over time, a more consistent sewn seam or stitch.
In addition to the amount and uniformity of the draw-off tension, controlling bobbin backlash or overspinning when sewing stops is another critical bobbin characteristic. Specifically, when the thread ceases to be pulled from the bobbin case, the bobbin in the bobbin case must not continue to spin and unravel to the point that the thread loops under the bobbin or over the bobbin case post which can snag and break when sewing is resumed. Even if the bobbin thread does not loop over or under the bobbin and snag after it overspins, the resulting slack created in the bobbin case can cause seam quality defects due to the subsequent tension variation once sewing is resumed.
Primary methods of controlling bobbin backlash or overspinning include the flanges on sidewall bobbins as well as the amount of tack on sideless bobbins. For example, the flanges of a sidewall bobbin act as a braking mechanism to help reduce the amount of backlash or overspinning when sewing stops. In addition, the flanges of a sidewall bobbin prevent the thread from looping over or under the bobbin. The amount of tack on a sideless bobbin will help reduce the amount of backlash or overspinning when sewing stops. Generally, the higher the bobbin tack level, the less overspinning or backlash. Assuming the proper amount and uniformity of draw-off tension, it is generally known in the art that the least amount of bobbin overspin will produce a more consistent sewn seam due to less tension variations and reduce the possibility of bobbin thread breaks.
While lockstitch sewing can create a very strong seam with low bulk that will not unravel, the disadvantage of lockstitch sewing is that you are limited to relatively short production runs before the underthread in the bobbin case runs out and needs to be replaced. Thus, the more yards that are provided on a given type and size of pre-wound bobbin, the fewer bobbin changes will be required which results in higher productivity--i.e., the sewing machine spends less unproductive down time to replace the bobbin and more production time is spent actually sewing. For example, assuming similar thread type and size, a pre-wound sideless bobbin will have more yards per bobbin than a bobbin with sidewalls. This is because a particular bobbin case can hold a bobbin with a maximum width or thickness and diameter. Without the need to have sidewalls that add to the width of the bobbin, a sideless bobbin can hold more yarn in a given bobbin case when compared to a pre-wound sidewall bobbin. This can have a positive impact on sewing productivity by reducing the number of times a bobbin must be changed as well as reducing the likelihood that the bobbin yarn will run out in the middle of a sewing pattern.
However, while sideless bobbins offer greater sewing productivity and less potential scrap, it is difficult to match the draw-off tension uniformity of a sidewall bobbin without creating excessive overspin. One must reduce the amount of tack to manufacture a sideless bobbin with a draw-off tension uniformity comparable to a sidewall bobbin. However, as the tack is reduced to improve the draw-off uniformity, the overspin of the sideless bobbin will increase. Conversely, if one increases the tack on the sideless bobbin to reduce any bobbin overspin then the draw-off tension becomes less uniform. This is because the yarn is tacked to an uneven underlying surface as each layer of yarn or thread is tacked to the previous layer. Thus, the more the tack level is increased to prevent or reduce overspin, the greater is the variation in bobbin draw-off uniformity.
Automatic bobbin changing equipment that loads the bobbin case with the full bobbin and unloads the bobbin case with the empty (or partially used) core has more recently attained widespread practice in the industry. Such automatic bobbin changing equipment that loads the bobbin case with the pre-wound bobbin installed in the bobbin case and unloads the bobbin case with the empty core or partially used bobbin requires that the bobbin remain securely in place in the bobbin case during the automatic loading and unloading operation. Pre-wound sidewall bobbins can be used in this automatic bobbin changing equipment as the mechanical finger on the bobbin case presses against the sidewalls during loading and unloading, thus securely holding the bobbin in the bobbin case during transfer by the automated bobbin changing equipment. However, conventional pre-wound sideless bobbins cannot be employed in automatic bobbin changing equipment since they do not have a sidewall.
The ability of longer yardage pre-wound sideless bobbins to stay securely in the bobbin case during automatic bobbin changing would have a positive impact on sewing productivity by reducing the number of bobbin changes and increasing the potential length of a continuous sewn pattern. In light of the above there is a need for a pre-wound high yardage sideless (with no sidewalls) bobbin that can be used in combination with automated bobbin changing equipment, and/or provides a more uniform draw-off tension throughout the bobbin while at the same time reducing the amount of overspin when sewing stops. It is toward fulfilling such a need that the present invention is directed.